Various types of magnetic signal transducers have been proposed in which a magnetic circuit is established including a permanent magnet, an inductive coil, a stator and a rotor, the stator and rotor, each, having projecting teeth which, upon rotation of the rotor, change the magnetic reluctance path and thereby change the magnetic flux through the pick-up coil. As the projecting teeth leave mutually engaged positions, a sharp needle pulse is induced in the pick-up coil which is used to control generation of a spark in the spark plug of the engine system. Transducers of this type are described in the literature, see, for example, German Patent Disclosure Document DE-OS No. 24 50 752. Signal sources of this type should be so constructed that they are compatible with the structures of mechanical breaker systems for installation in existing distributor housings of well known and standard types. The signal sources are, thus, installed in a combined structure functioning both as an ignition signal source and as a distributor to distribute the spark energy to respective ones of various cylinders in a multi-cylinder engine. Additionally, the distributor-signal source unit includes means to adjust the ignition timing. The ignition timing apparatus usually is so designed that the ignition timing changes in dependence on speed of the internal combustion engine, and, additionally, may have a further adjustment which is coupled to the intake manifold or induction pipe of the engine to change the ignition timing in dependence on vacuum therein. The various ignition timing adjustment elements, of standard and well known construction, are effective only after the engine has reached a certain speed, above the minimum speed upon, or shortly after, starting. Thus, adjustment of ignition timing by using the well known vacuum diaphragm adjustment and, for example, centrifugal adjustment, does not permit adjusting of the ignition spark to delayed or retarded operation, as would be desirable upon starting and when the engine is running immediately thereafter. Adjusting the spark towards retardation substantially facilitates starting of the engine since the spark plug can then initiate ignition of the air-fuel mixture reliably only after the piston has already passed the upper or top dead center (TDC) position.